1. Field of the Invention
The present invention relates to a method of producing a thermotropic liquid crystalline copolyester which the amount of corrosive out-gases emitted in a high temperature environment is extremely small, a thermotropic liquid crystalline copolyester resin composition obtained by the method, and a resin molded article made of the same resin composition for use in electrical/electronic components. More specifically, the present invention relates to a method of producing a thermotropic liquid crystalline copolyester which the amount of corrosive out-gases (such as acetic acid and phenol) emitted in a high temperature environment is extremely small due to the setting of the amount of acetic anhydride in the reaction system in which acetylation is carried out before polymerization to a specific range, a thermotropic liquid crystalline copolyester resin composition which the amount of corrosive out-gases (such as acetic acid and phenol) emitted in a high temperature is further extremely small and is made by mixing a specific phosphate compound to a thermotropic liquid crystalline copolyester resin obtained by the same method, and a resin molded article made of the same resin composition for use in electrical/electronic components.
2. Description of the Related Art
It has been recognized that thermotropic liquid crystalline copolyesters made by known methods tend to emit corrosive out-gases which corrode metal-made conductive portions (e.g. an electronic circuit) of an electric/electronic component in a high temperature environment (such as soldering and mounting-to-surface processes). Corrosiveness of such corrosive out-gases has been recognized as a serious problem in such cases. Studies have revealed that the main component of such corrosive out-gases is generally acetic acid (refer to, for example, JP-A 8-53543).
Specifically, in electric/electronic components having metal-made conductive portions which is vulnerable to the gases emitted from a thermotropic liquid crystalline copolyester resin (such as a relay, a switch, a connector, a socket, a resistor, a condenser, a motor, an oscillator, a print circuit board, and a power module), the metal-made conductive portions are oxidized and a corrosive film is formed thereon by the corrosive out-gases and the like due to heat history during the mounting-to-surfaces process. As a result, failure in the conductive portions may occur. In addition, in a case in which the electrical/electronic component has an electric contact which is operated in a mechanical manner, a failure in contact may occur due to formation of layers of carbonized materials in the contact portion (the layers are formed mainly in the contact portion by discharge during the contact operation).
The corrosion of this type has particularly been a serious problem in components such as a relay and a switch in which good contact properties must be maintained for a long period.
Recently, thermotropic liquid crystalline copolyesters are also used in various components in HDD (e.g. a carriage, a chassis and a VCM coil holding member for an actuator, a member for installing a head in a non-operationphase and the like), in FDD and in similar components in an optical disc drive and the like. With respect to the magnetic or optical data reading portions which are essential to these devices, deterioration of performances due to corrosive out-gases emitted from the resin are now likewise being concerned.
As thermotropic liquid crystalline copolyesters can be molded so as to have thin walls (i.e. these copolyesters have excellent molding/fluxional properties) and have excellent soldering properties (i.e. these copolyesters have excellent heat resistance properties), they have been employed as forming materials of various electric/electronic components so that excellent dimensional precision obtained in the copolyesters be most advantageously utilized. In addition, the electric/electronic components are now required to be far smaller and operated at a lower voltage. Accordingly, formation of corrosive film and generation of layers of carbonized materials as described above could cause much worse, more often initial failures or malfunction in these electric/electronic components than now. Therefore, there is a demand for a thermotropic liquid crystalline copolyester which the amount of corrosive gases is extremely small. This may be especially a concern in a relay component and a switch component. Note that the layers of carbonized materials are formed in these components probably because the corrosive out-gases are carbonized by arc discharge and deposited, causing abnormality in conductance.
As methods for reducing corrosive out-gases from themotropic liquid crystalline copolyester, there have been proposed a method of blending a gas absorbing material (JP-A 8-333505), a method of blocking the end of the molecular chain with mono-functional monomer (JP-A 3-203925, JP-A 4-249528 and JP-A 8-53543). However, these methods are not necessarily satisfactory.
These conventional methods propose, assuming that the main component of the corrosive gases is acetic acid emitted from the thermotropic liquid crystalline copolyester, techniques for suppressing the generation of acetic acid and capturing the generated acetic acid. However, it has not been determined what actually are the corrosive out-gases which cause corrosive damages to metal-made conductive portions of electric/electronic components. Therefore, although emission of acetic acid is prevented, it does not necessarily mean that a thermotropic liquid crystalline copolyester which is satisfactory in terms of its corrosive out-gas effect on an electric/electronic component can be obtained. Especially, if the technique pays too much attention to suppression of acetic acid emission and rather increases emission of other corrosive out-gases, such technique or methods inevitably have to face a serious limitation.
With respect to this problem, the inventors have discovered that thermotropic liquid crystalline copolyester may emit phenol, which is corrosive and could be carbonized, together with acetic acid in a high temperature environment. Based on this discovery, the inventors were convinced that a thermotropic liquid crystalline copolyester which the amount of corrosive out-gases is very small and thus can be used as a reliable forming material for an electric/electronic component (in other words, a thermotropic liquid crystalline copolyester which satisfies the demand from an electric/electronic component) is effected by suppressing the generation of phenol. The present invention was completed as a result of industrious study according to this theory.
The detailed mechanism in which corrosive out-gases are emitted from thermotropic liquid crystalline copolyester is not known yet. The inventors, however, discovered for the first time in the world that the amount of emission of both corrosive out-gases (acetic acid and phenol) can be suppressed by setting the amount of acetic anhydride in the reaction system in which acetylation is carried out before polymerization within a specified range, resulting in the present invention.
Generation of corrosive gases tends to be accelerated by the existence of inorganic or organic fillers blended into the copolyester. In the case of engineering plastics such as thermotropic liquid crystalline copolyester, inorganic or organic fillers are normally blended in practice. Accordingly, it is required that generation of corrosive gases be reliably suppressed in the resins in which inorganic or organic fillers are blended.
The inventors of the present invention have achieved reliably suppressing generation of out-gases at a practically acceptable level in the resin compositions in which fillers are blended, by adding a specific phosphate compound into a thermotropic liquid crystalline copolyester obtained by the aforementioned method.